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Chapter 12
The Cell Cycle
“Every cell from a cell”
--- the continuity of life is based one the reproduction
of cells, or cell division.
Cell cycle:
--- the life of a cell from the time it is first formed from a
dividing parent cell until its own division into two cells.
Cell division functions in reproduction, growth, and renewal.
200 µm
20 µm
Cell division:
--- involves the distribution of identical genetic materialDNA-to two daughter cells.
Genome 基因體
--- genetic information, a cell’s endowment of DNA.
--- human cell ~ 2 m of DNA
Chromosomes 染色體
--- every eukaryotic species has a characteristic number of
chromosomes in each cell nucleus.
* Somatic cells have two sets of chromosomes
* Gametes have one set of chromosomes
Chromatin (染色質)
Eukaryotic chromosomes are made of chromatin, a complex
of DNA and associated protein molecules.
Gene (基因) :
the units that specify and organism’s inherited traits.
chromatin
~ long, thin fiber
duplication
chromatin condense
chromosome
Chromosome
duplication
Chromatin
DNA-protein complex
Sister chromatids :
containing identical copies of the chromosome’s DNA.
0.5 µm
A eukaryotic cell has multiple
chromosomes, one of which is
represented here. Before
duplication, each chromosome
has a single DNA molecule.
Once duplicated, a chromosome
consists of two sister chromatids
connected at the centromere. Each
chromatid contains a copy of the
DNA molecule.
Mechanical processes separate
the sister chromatids into two
chromosomes and distribute
them to two daughter cells.
Chromosome
duplication
(including DNA
synthesis)
Centromere
Separation
of sister
chromatids
Centromeres
Sister
chromatids
Sister chromatids
* Mitosis (有絲分裂)
--- the division of the nucleus.
* Cytokinesis (細胞質分裂)
Two set of
chromosome
--- the division of the cytoplasm.
* Meiosis (減數分裂)
--- yields daughter cells that
have half chromosomes.
one set of
chromosome
Mitosis is just one part of the cell cycle.
Mitotic (M) phase : includes both mitosis and cytokinesis
(the shortest part of the cell cycle)
* A typical human cell might undergo one division in 24 hours.
* G1 is the most variable
in length in different
90%
5~6 hr
types of cells.
10~12 hr
4~6 hr
less than 1 hr
The mitotic division of an animal cell
Interphase:
Prophase prometaphase
G2
metaphase
telophase &
anaphase cytokinesis
Longest stage
shortest stage
The mitotic spindle distributes chromosomes to daughter cells.
(紡錘絲)
~ begins to form in the cytoplasm during prophase.
~ * made of microtubules and associated proteins.
* elongated by incorporating more subunits of the protein tubulin.
* starts in the centrosome. (microtubule-organizing center; MTOC)
* centrioles are not essential
for cell division.
* Spindle includes the
centrosomes, the spindle
microtubules, and the asters.
* Kinetochore:
~ a structure of proteins
associated with specific
sections of chromosomal
DNA at the centromere.
* Kinetochore microtubules
* Nonkinetochore microtubules
(“polar” microtubules)
Aster
Sister chromatids
Centrosome
Metaphase
Plate
Kinetochores
Overlapping
nonkinetochore
microtubules
Kinetochores microtubules
Microtubules
* Microtubules of asters
0.5 µm
Metaphase
--- the spindle complete
 contact with the plasma
membrane (in metaphase)
Centrosome
1 µm
Chromosomes
Metaphase
Anaphase
Anaphase commences when proteins holding together the
sister chromatids of each chromosome are inactivated
How do the kinetochore microtubules function
in the poleward movement of chromosomes ?
Kinetochores are equipped with motor proteins that
“walk” a chromosome along the attached microtubules
toward the nearest pole.
The microtubules shorten by depolymerizing at their kinetochore ends.
What’s the function of the nonkinetochore microtubules ?
--- for elongating the whole cell during anaphase.
* Nonkinetochore microtubules interdigitate across the metaphase plate
Anaphase:
1. Nonkinetochore microtubules orininating from opposite
spindle poles move past each other toward their poles.
2. The nonkinetochore microtubules lengthen by the
addition of tubulin subunits to their ends.
Cytokinesis divides the cytoplasm
* Cleavage furrow
Actin
+
Myosin
Cleavage furrow
Contractile ring of
microfilaments
* No cleavage furrow
100 µm
Vesicles
forming
cell plate
Wall of
patent cell
1 µm
Cell plate
New cell wall
Daughter cells
Daughter cells
(a) Cleavage of an animal cell (SEM)
(b) Cell plate formation in a plant cell (SEM)
• Mitosis in a plant cell
Chromatine
Nucleus
Nucleolus condensing
1 Prophase.
The chromatin
is condensing.
The nucleolus is
beginning to
disappear.
Although not
yet visible
in the micrograph,
the mitotic spindle is
staring to from.
Figure 12.10
Chromosome
2 Prometaphase.
3 Metaphase. The
4
We now see discrete
spindle is complete,
chromosomes; each
and the chromosomes,
consists of two
attached to microtubules
identical sister
at their kinetochores,
chromatids. Later
are all at the metaphase
in prometaphase, the
plate.
nuclear envelop will
fragment.
Anaphase. The
5
chromatids of each
chromosome have
separated, and the
daughter chromosomes
are moving to the ends
of cell as their
kinetochore
microtubles shorten.
Telophase. Daughter
nuclei are forming.
Meanwhile, cytokinesis
has started: The cell
plate, which will
divided the cytoplasm
in two, is growing
toward the perimeter
of the parent cell.
Prokaryotes (bacteria) reproduce by a type of cell division
called binary fission, meaning literally “division in half”.
Cell wall
Origin of
replication
E. Coli cell
1 Chromosome replication begins.
Soon thereafter, one copy of the
origin moves rapidly toward the
other end of the cell.
2 Replication continues. One copy of
the origin is now at each end of
the cell.
3 Replication finishes. The plasma
membrane grows inward, and
new cell wall is deposited.
4 Two daughter cells result.
Two copies
of origin
Origin
Plasma
Membrane
Bacterial
Chromosome
Origin
Mitosis in eukaryotes may have evolved from binary fission
in bacteria.
* A hypothetical sequence
for the evolution of mitosis
Prokaryotes.
Bacterial
chromosome
Chromosomes
Microtubules
~ Certain protists exhibit types
of cell division that seem
intermediate between binary
fission and mitosis carried
out by most eukaryotic cells
Dinoflagellates.
(腰鞭毛蟲)
Intact nuclear
envelope
Kinetochore
microtubules
Diatoms.
(矽藻)
Intact nuclear
envelope
Kinetochore
microtubules
Most
eukaryotes.
Centrosome
Fragments of
nuclear envelope
Concept 12.3: The cell cycle is regulated by a molecular control system
The frequency of cell division varies with the type of cell
Ex. Human skin cells: divide frequently throughout life.
liver cells: keep in reserve (to repair a wound)
mature nerve cells and muscle cells: do not divide at all.
These cell cycle differences
~ Result from regulation at the molecular level
What drives the cell cycle ?
The cell cycle is driven by specific chemical signals
present in the cytoplasm.
Experiment 1
Experiment 2
S
S
G1
S
the G1 cell immediately
entered the S phase—
DNA was synthesized.
M
M
G1
M
the G1 cell immediately began mitosis— a
spindle formed and chromatin
condensed, even though the chromosome
had not been duplicated.
The sequential events of the cell cycle are directed by
a distinct cell cycle control system, a cyclically
operating set of molecules in the cell that both triggers
and coordinates key events in the cell cycle.
~ similar to a clock
G1 checkpoint
Control
system
G1
M
G2
M checkpoint
G2 checkpoint
S
The cell cycle is
regulated at certain
checkpoints by both
internal and external
controls.
Cell cycle “checkpoints”
* A checkpoint is a critical control point where stop and
go-adhead signals can regulate the cycle.
(~ signal transduction pathways)
Animal cells generally have built-in stop signals that halt the
cell cycle at checkpoints until overridden by go-ahead signals.
* G1, G2, and M phase checkpoint
* G1 checkpoint: (restriction point)
~ the most important checkpoint.
* G1 checkpoint: (restriction point)
x
Go-ahead signal
Complete the cell cycle and divide.
Exit the cycle, switching into a
nondividing state (“G0 phase”)
G0
G1 checkpoint
G1
G1
(a) If a cell receives a go-ahead signal (b) If a cell does not receive a go-ahead signal at
the G1checkpoint, the cell exits the cell cycle
at the G1 checkpoint, the cell
and goes into G0, a nondividing state.
continues on in the cell cycle.
What kinds of molecules make up the cell cycle control
system (the molecular basis for the cell cycle clock)?
Two types of regulatory proteins are involved in cell
cycle control:
Cyclins and cyclin-dependent kinases (Cdks)
constant
concentration
inactive
cdk
active
cdk
cyclin
cyclically fluctuating
concentration
The activity of cyclins and Cdks
~ Fluctuates during the cell cycle
~ Ex. MPF “ maturation-promoting factor”
“M-phase-promoting factor”
~ first identified cyclin-cdk complex
~ triggers the cell’s passage past the G2 checkpoint into M phase.
Cdk
Degraded
Cyclin
Cyclin is
degraded
G2
checkpoint
MPF
Cdk
Cyclin
Stop and Go Signs:
Internal and External Signals at the Checkpoints
• Both internal and external signals
– Control the cell cycle checkpoints
Internal signals: originating inside the cell
~ ex. Messages from the kinetochores
Anaphase onset:
When the kinetochores of all the
(M phase checkpoint) chromosomes are attached to the spindle
Breakdown of cyclin and the inactivation of
proteins holding the sister chromatids together.
Sister chromatids separate
External signals: ex. Growth factors
~ Cells fail to divide if an essential nutrient is left out of the
culture medium.
~ GFs trigger a signal transduction pathway that allows the
cells to pass the G1 checkpoint and divide.
PDGF
PDGF
receptor
cell
Signal transduction
Cell division
External signals: physical factor
Density-dependent inhibition of cell division
~ Crowded cells stop
dividing
single layer
Cells anchor to dish surface and
divide (anchorage dependence).
When cells have formed a complete
single layer, they stop dividing
(density-dependent inhibition).
If some cells are scraped away,
the remaining cells divide to fill
the gap and then stop (densitydependent inhibition).
25 µm
• Most animal cells exhibit anchorage dependence
– In which they must be attached to a substratum to
divide
Anchorage dependence
* Cancer cells:
~ Exhibit neither density-
dependent inhibition nor
Normal cell ~ single layer
Cancer cells do not exhibit anchorage
dependence or density-dependent inhibition.
anchorage dependence
25 µm
25 µm
Chapter 13
Meiosis and Sexual Life Cycles
The transmission of traits from one generation to
the next is called inheritance, or heredity. (遺傳)
~ Genetics
• Overview: Hereditary Similarity and Variation
• Living organisms
–
Are distinguished by their ability to reproduce their
own kind
• Heredity
–
Is the transmission of traits from one generation to the next
• Variation
–
Shows that offspring differ somewhat in appearance from
parents and siblings
Figure 13.1
Offspring acquire genes from parents by inheriting
chromosomes.
~ Parents endow their offspring with coded information in
the form of hereditary units called genes.
 constitute our “genome”
Genes are segments of DNA.
~ DNA is a polymer of four different kinds of monomersnucleotides. (核苷酸)
Asexual and sexual reproduction
(無性)
(有性生殖)
In asexual reproduction,
~ a single individual is the sole parent and passes copies of all its
genes to its offspring. (the genomes of the offspring are
virtually exact copies of the parent’s genome)
“clone” (a group of genetically identical individuals)
In sexual reproduction,
~ offspring vary genetically from their
siblings and both parents.
Hydra, 水螅
~ reproduce by budding.
~ usually genetically
identical to its parent.
Parent
Bud
0.5 mm
Somatic cell
Gametes
(體細胞)
(sperm or ovum)
~ 46 chromosomes
~ 23 chromosomes
With a light microscope, condensed (mitotic) chromosomes can
be distinguished from one another by their appearance.
Differ from ~ size, the positions of their centromeres
~ staining pattern
* Karyotype
~ the images of the chromosomes are arranged in
pairs, starting with the longest chromosomes.
Karyotypes
~ ordered displays of an individual’s chromosomes.
Pair of homologous
chromosomes
Centromere
Sister
chromatids
5 µm
Karyotyping can be used to screen for abnormal numbers of
chromosomes or defective chromosomes associated with
certain congenital disorders, such as Down syndrome.
Down syndrome: each body cell has a total of 47 chromosomes.
(cells are trisomic for chromosome 21; trisomy 21)
The chromosomes that make up a pair-that have the same
length, centromere position, and staining pattern
-are called homologous chromosomes, or homologues.
carry genes controlling the same inherited characters.
Key
A gene’s specific location
along the length of a
chromosome is called the
gene’s locus.
Maternal set of
chromosomes (n = 3)
2n = 6
Paternal set of
chromosomes (n = 3)
Two sister chromatids
of one replicated
chromosome
Centromere
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)
Exception of the homologous chromosome:
X & Y chromosome:
Only small parts of the X and Y
are homologous.
Female: XX
Male: XY
sex chromosomes
autosomes
The occurrence of homologous pairs of chromosomes in our
karyotype is a consequence of our sexual origins.
46 chromosome --- two sets of 23 chromosomes.
~ a maternal set (from mother) and
a paternal set (from father)
Somatic vs. gametes
A diploid cell.
(2n=46)
a single set of the 22 autosomes
+
a single sex chromosome
A haploid cell.
(n=23)
Sperm + ovum
fertilization
Fertilized egg (zygote)
• The human life cycle
Key
The processes of
Haploid gametes (n = 23)
Haploid (n)
Diploid (2n)
Ovum (n)
fertilization and meiosis
are the unique trademarks
Sperm
Cell (n)
of sexual reproduction.
FERTILIZATION
MEIOSIS
~ alternate in sexual life
cycles.
Ovary
Testis
Mitosis and
development
Figure 13.5
Multicellular diploid
adults (2n = 46)
Diploid
zygote
(2n = 46)
• In animals
– Meiosis occurs during gamete formation
– Gametes are the only haploid cells
Key
Haploid
Diploid
n
n
Gametes
n
MEIOSIS
FERTILIZATION
Zygote
2n
Figure 13.6 A
Diploid
multicellular
organism
2n
Mitosis
(a) Animals
• Plants and some algae
–
Exhibit an alternation of generations
–
The life cycle includes both diploid and haploid
multicellular stages
Alternation of generations
Haploid multicellular
organism (gametophyte)
~ includes both diploid and
haploid multicellular stages.
n
Mitosis
n
Mitosis
n
n
n
Spores
Gametes
MEIOSIS
Diploid
multicellular
organism
(sporophyte)
Figure 13.6 B
FERTILIZATION
2n
(b) Plants and some algae
2n
Mitosis
Zygote
• In most fungi and some protists
– Meiosis produces haploid cells that give rise to a haploid
multicellular adult organism
– The haploid adult carries out mitosis, producing cells
that will become gametes
Haploid multicellular
organism
n
Mitosis
Mitosis
n
n
n
Gametes
MEIOSIS
FERTILIZATION
2n
Figure 13.6 C
Zygote
(c) Most fungi and some protists
n
Meiosis reduces chromosome
number from diploid to haploid.
• Interphase and meiosis I
MEIOSIS I: Separates homologous chromosomes
INTERPHASE
PROPHASE I
METAPHASE I
ANAPHASE I
2. cross over
Sister
chromatids
Nuclear
envelope
Chromatin
Chromosomes duplicate
Figure 13.8
Sister chromatids
remain attached
Centromere
(with kinetochore)
Centrosomes
(with centriole pairs)
Tetrad
Chiasmata
Spindle
Metaphase
plate
Homologous
Microtubule
chromosomes
attached to
separate
kinetochore
Tertads line up
Homologous chromosomes
(red and blue) pair and exchange
segments; 2n = 6 in this example
1. Synapsis (聯會)
(synaptonemal complex)
Pairs of homologous
chromosomes split up
• Telophase I, cytokinesis, and meiosis II
MEIOSIS II: Separates sister chromatids
TELOPHASE I AND
CYTOKINESIS
PROPHASE II
Cleavage
furrow
Figure 13.8
Two haploid cells
form; chromosomes
are still double
METAPHASE II
ANAPHASE II
Sister chromatids
separate
TELOPHASE II AND
CYTOKINESIS
Haploid daughter cells
forming
During another round of cell division, the sister chromatids finally separate;
four haploid daughter cells result, containing single chromosomes
• A comparison of mitosis and meiosis
MITOSIS
MEIOSIS
Parent cell
(before chromosome replication)
Chiasma (site of
crossing over)
MEIOSIS I
Prophase I
Prophase
Chromosome
replication
Duplicated chromosome
(two sister chromatids)
Chromosome
replication
Tetrad formed by
synapsis of homologous
chromosomes
2n = 6
Metaphase
Chromosomes
positioned at the
metaphase plate
Anaphase
Telophase
Sister chromatids
separate during
anaphase
2n
Tetrads
positioned at the
metaphase plate
Homologues
separate
during
anaphase I;
sister
chromatids
remain together
Metaphase I
Anaphase I
Telophase I
Haploid
n=3
Daughter
cells of
meiosis I
2n
MEIOSIS II
Daughter cells
of mitosis
n
n
n
n
Daughter cells of meiosis II
Sister chromatids separate during anaphase II
A Comparison of Mitosis and Meiosis
• Meiosis and mitosis can be distinguished from mitosis
– By three events in Meiosis l
1. Synapsis (聯會) and crossing over (交換)
– Homologous chromosomes physically connect and
exchange genetic information
2. Tetrads on the metaphase plate
– At metaphase I of meiosis, paired homologous chromosomes
(tetrads) are positioned on the metaphase plates
3. Separation of homologues
– At anaphase I of meiosis, homologous pairs move toward
opposite poles of the cell
– In anaphase II of meiosis, the sister chromatids separate
Chiasmato are the physical manifestations of a genetic
arrangement called crossing over.
Origins of genetic variation
Three mechanisms that contribute to the genetic
variation arising from sexual reproduction:
1. Independent assortment of chromosomes
2. Crossing over
3. Random fertilization
1. Independent assortment of chromosomes
Key
Maternal set of
chromosomes
Paternal set of
chromosomes
Possibility 1
Possibility 2
Two equally probable
arrangements of
chromosomes at
metaphase I
2n
(n = the haploid
number)
Metaphase II
223
Daughter
cells
Combination 1
Combination 2
Combination 3
Combination 4
2. Crossing over
Prophase I
of meiosis
Nonsister
chromatids
The process called crossing
Tetrad
over produces recombinant
Chiasma,
site of
crossing
over
chromosomes, which combine
genes inherited from our
Metaphase I
two parents.
Metaphase II
Daughter
cells
Recombinant
chromosomes
3. Random fertilization
Sperm + ovum
fertilization
223
223
Fertilized egg (zygote)
223 X 223
+
Cross over
Evolutionary adaptation depends on a population’s
genetic variation
Darwin
~ recognized the importance of genetic variation in the
evolutionary mechanism ~ natural selection.
Adaption, the accumulation of the genetic variations
favored by the environment.
Sex and mutations are the two sources of this variation
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